This invention relates to a virtual impactor type aerosol concentration or collection device.
Virtual impactor particle collection devices are well-known, being described, for example in U.S. Pat. Nos. 4,767,524, 6,010,554, 6.120,573 and 6,290,065, among others. Generally, these consist of an acceleration nozzle, in which the flow path is constricted in order to increase flow velocity, and a separation zone located downstream from the acceleration nozzle, where the fluid stream is split into at least two channels. A minor (in terms of fluid volume) flow travels more or less in the original direction of flow (i.e., the direction of flow through the constriction). The inertia of the particles causes most of them to be carried off with this minor flow. A major flow containing most of the fluid is taken off at an angle to the original direction of flow. The large inertia of the particles makes them less able to navigate this change in direction, and so this major flow contains a smaller particle concentration than either the original stream or the minor flow. The fluid containing the concentrated particles is then passed through a collector which removes the particles from the fluid. Virtual impactor type concentrators often have the further advantages that they require relatively low pressure drops, can be quite small in size, have low energy requirements, and are scalable.
Virtual impactor devices are highly prone to fouling due to collisions of the particles with the side walls of the device. These collisions can occur in the acceleration nozzle, in the separation zone, or both. The fouling problem is often so great that the virtual impactor device effectively functions like a filter, removing rather than concentrating the particles. As a result, many virtual impactor devices exhibit a concentration efficiency, defined as the ratio of particle concentration in the inlet fluid to that of the (supposedly) concentrated exit stream, of less than 1.0. So many of the particles become captured by the virtual impactor device that the exit streams often contain a lower concentration of particles than the starting fluid.
It is highly desirable to provide a virtual impactor device which is less prone to fouling and produces an effluent stream with a higher concentration of particles.